Every time you install a new mobo, faulty or otherwise, you have to reinstall Windows. This is because the instalation will pull down all the drivers approrpiate to what's on that mobo. So your option will not work. However, yopu could try also attahcing the HD to your PC - that way, the correct OS will be running.
In any case, you don't need to worry about activation if you only need it for a short while.